The invention concerns a driver for the transfer of torque and rotation to the drill steel in a rock drilling machine. The driver is an exchangeable part subject to wear, and has been designed for mounting in the gear housing of the rock drilling machine, where its task is to transfer torque and rotation from a rotation chuck to the drill steel. The drill steel can be displaced axially in the driver during the transfer. The driver is for this purpose externally provided with a polygonal profile and internally provided with splines. The polygonal profile and the splines are in the present invention arranged in an innovative manner that increases the lifetime of the driver and reduces the risk of fatigue failure. The invention concerns also a rock drilling machine that comprises at least one such driver.
FIG. 1 shows a reduced section of a prior art gear housing 1 of a rock drilling machine. The upper part, known as the shank adapter, of a drill steel 2 can be seen in the centre of the gear housing 1. The shank adapter is externally provided with splines. The shank adapter is inserted into a known driver 3 that is provided with internal splines 4 that correspond to the splines of the shank adapter. The driver 3 is provided with an external polygonal profile and is mounted into a rotation chuck 5 that has an internal polygonal profile that corresponds to the polygonal profile of the driver 3. The rotation chuck 5 is arranged on bearings in the gear housing 1 and is rotated by the rotational motor of the rock drilling machine through a gear (not shown in the drawing). The torque that is transferred may amount to 1000 Nm or more.
The cross-section of the driver 3 is limited outwards by curves, united to form a closed FIG. 6 that is approximately described by a polygon with three sides 7 and three corners 8. This type of united figure is normally known as a “polygon profile”. The FIG. 6 is transversed by three imaginary lines 9 of symmetry, drawn from the centre of the driver 3 and passing through the corners 8 of the FIG. 6 such that these are symmetrically distributed around the lines 9 of symmetry.
The cross-section of the driver 3 is limited inwards by the splines 4 with eight spline teeth 10 and eight spline spaces 11. The drawing makes it clear that it is possible to arrange only one of the eight spline spaces 11 symmetrically around any one of the three lines 9 of symmetry. When one of the spline spaces 11 is arranged symmetrically around the uppermost of the lines 9 of symmetry, two of the spline teeth 10 acquire an asymmetric distribution around the two lower lines 9 of symmetry. Similar results would have been obtained if one of the spline teeth 10 had been initially taken. The asymmetry leads to certain parts being subject to considerably higher load than other parts of the driver 3.
FIG. 2 shows an enlarged cross-section of one of the spline spaces 11 of the prior art driver 3, the shank adapter of the drill steel 2 and the rotation chuck 5 from FIG. 1. The direction of rotation of the rotation chuck 5 is shown by an arrow at the bottom. The driver 3 is arranged such that the ratio between the diameter d2 of the bottom circle of the spline 4 and the diameter d1 of the top circle is 1.38. The diameter D2 of the top circle of the spline teeth of the shank adapter is adapted such that it constitutes approximately 98% of the diameter d2 of the bottom circle of the splines 4 of the driver 3. The arrangement and the contact pressure between the spline teeth 10 of the shank adapter and the driver 3 lead to small amounts of the material of the driver 3 being pressed down to the root radii 12 of the spline spaces 11 during the transfer of torque. The material that is pressed down constitutes fracture notches or burrs 13 along the radii 12 and impairs in this way its function of protecting from fatigue failure. The drawing shows how these burrs 13 initiate the formation of cracks in the root radii 12.